Microwave device



D. H. SLOAN MICROWAVE DEVICE Jgne 9, 1953 3 Sheets-Sheet l Filed Faut. 5. 1947 June 9, 1953 D. H. SLAN 2,641,734

MICROWAVE DEVICE Filed sept. 5. 194'? 3 sheets-Sheet 2 June 9, 1953 DH. SLOAN 2,641,734

MICROWAVE DEVICE Filed Sept. 3, 1947 5 Sheets-Sheet 3 if F16. 4

` 3f? 4f j y r d 3X' 40 3 7 if j w /45 r V\ M l /ff i /49 //7 77 2f J/ f/ 7/ /lVVi/VTOE @A wp .54 amv @IMQ/KW Patented June 9, 1953 MICROWAVE DEVICE v David H. Sloan, Berkeley, Calif., assignor to Research Corporation, New York, N. Y., a corporation of New York Application September 3, 1947, Serial No. 771,852

(Cl. S15- 39) 13 Claims.

This invention relates broadly to resonators for microwave apparatus, i. e., to apparatus for operation at frequencies corresponding to wave lengths of materially less than one meter. Such resonators find their widest application in tubes for generating and amplifying such frequencies, and the specific embodiments of the invention described and claimed herein relate to such tubes, although other applications will suggest themselves to those skilled in the art.

The art of generating microwaves of high power received great impetus through the development of cavity resonators; prior to this development the small physical size of the structures which would respond to the frequencies of the waves imposed severe limitations on the currents they would carry without undue heating. Cavity resonance permitted larger structures to be used and greatly increased the power which could be generated. The demands for higher and higher powers, however, exceed the capacities of the resonators which can now be termed conventional.

One of the major difficulties in supplying these demands is that a cavity will resonate in a theoretically infinite number of modes, and these modes are not integral multiples of a single fundamental, as is the case with older types of structure, but may be of quite closely adjacent frequencies. Very minor diiferences in dimension, in placement of elements or in the exciting voltage applied, may determine what the mode or modes of oscillation may be, for the device may oscillate simultaneously at a number of different frequencies, and in such case all but one of the modes will be parasitic, withdrawing power from the desired mode and wasting it as heat.

Accordingly it has been considered necessary to make the resonating structures symmetrical, to locate the excitation on the axis, and'to make the transverse dimension of the cavity not greater than one wave length of the desired mode, which, in general, is the principal mode, or mode of lowest frequency. These rules may bemodifled in multiple cavity structures, but otherwise are quite generally followed,

The requirement for axial excitation tends to set a limit to the power that can be produced by a single oscillator. Where excitation is through the electric field the excitation should be at a voltage loop in this field, which is theoretically a point, the magnetic field being circular and with the voltage loop or maximum on the axis and the currents flowing radially, the current maximum and voltage node being at the circumference. Actually the exciting elements must have dimension, but this must be restricted so that they are confined within a space of less than one-half wave length diameter if this mode is to dominate, and with short wave length this seriously limits the size of the exciting elements and hence the power they can handle at a given voltage.

It is clear that if, instead of being confined to the neighborhood of a point, the potential loop at which excitation takes place were a circle of a wave length diameter the power could be enormously increased, but in the past this has been considered impossible, for it involves resonance of the cavity at other than its principal mode, and there are numerous other modes that are nearly, if not quite, as easy to excite as the desired one and hence which may absorb large amounts of power and render the operation of the device uncertain and unstable.

Broadly stated, the purpose of this invention is to provide means for generating, amplifying and otherwise handling microwaves of large power, up to 1000 kilowatts or more. Among the objects, when considered in detail, being to provide means for forcing the oscillation of a resonant cavity in a desired mode of oscillation; to provide a means of exciting a cavity oscillator at a single mode from positions other than axial; to provide a cavity oscillator which can be driven at a predetermined mode by a plurality of distributed exciting elements of relatively large aggregate area; to provide a cavity resonator in which troublesome `parasitic oscillations are suppressed; and to provide a type of vacuum tube which will generate microwaves of many kilowatts power or amplify such waves to give a like output.

Other objectives and features of this invention lwill become apparent by reference to the following description and the accompanying drawings wherein:

Fig. 1 is an axial sectional view of a vacuum tube amplifier embodying my invention;

Fig. 2 is a transverse sectional view of the same tube, the plane of section being indicated by the line 2-2 of Fig. 1;

Fig. 3 is a fragmentary section being a developed circumference, the trace of whose intersection with the plane of Fig. 1 is indicated by the line 3 3 thereon.

Fig. 4 is a cross sectional view of the cathode support column of the tube shown in Fig. 1, the plane of section being on the line 4 4 of Fig. 1;

Fig. 5 is an alternate cathode structure for a tube of the same type as shown in Fig. 1;

Fig. 6 is an enlarged fragmentary section taken through the central plane of the filament and v grid structure of Fig. 1; y

Fig. 7 yis a similar section to that of Fig. 6, showing the structure of cathodes and grids in the type of construction illustrated in Fig. 5.

From one point of view, this invention comprises a cavity resonator Whose lateral dimensions are larger than one Wave length of its desired frequency of oscillation; preferably the cavity is bilaterally symmetrical and its axial length is short as compared to said wave length, and conveniently it is circular, but the cavity may be of any of a multitude of forms; at and plate-like, the shell yof a cone, formed between concentric spheres or combinations of these or a plurality of exciting elements, which shouldVv also be spaced an odd number of quarter wave lengths from the periphery of the cavity; these dimensions are only approximate, however, since Y it is possible to so load the cavity astovary the length of the waves therein, and since the exciting elements are of finite size and need not be precisely centered at the half-wave distance, a.

considerable degree of tuning is possible.

Means are provided for suppressing some of the less desirable modes of oscillation of the cavity, or, at least, for insuring that if parasitic modes do occur they will be of definite character. Such means may be slots, which are parallel to the current flow in the principal mode but across the lines of current flow in the` less desired modes, formed in the plates of the cavity. Another method is to short-circuit the cavity at voltage nodes of the desired mode. iStill another method of accomplishing the purpose isto step the cavity, making its axial length greater in certain portions than in others,V thus causing irregularities which would have a suppressing effect on certain undesired modes, by destroying the symmetry of the cavity, but would have no material effect on Athe desired mode. Y

From another point of view, the broad invention resides in the tube itself, and not in the cavities, but in their coupling. The tube used is a tetrode, with. built-in grid and plate resonant cavities. These cavities are coupled, either electronically or through electrical or magnetic fields, or material electrical circuits. lation of the tube in its desired mode is assured by making the resonant cavities different insofar as their parasitic modes of oscillation is concerned, but, in their desired mode, responsive to the same frequency; in other. words, the cavities are turned with. respect to the desired modes but detuned from the undesiredones.

A. tetrode oscillator invention is illustrated in Fig. l.v This tube embodies the principles disclosed in the prior Patent No. 2,424,002, issued. July 15, 19.47, to Sloan, insofar as the tube elements are concerned, and reference is made to that patent for the theory of the'quarter-wave transmission line chokes and by-passes which are used throughout to prevent leakage and radiation of the high frequency power. These arrangements are particularly important in the present instance because of the large amounts of power generated by the tube, since the escape centage would` cause serious heating and interference.

In the detailed description of this tube the point of Vdeparture will be taken as the grid base I, and the supporty of the cathodes, grid, accelerating grid, and anode will be referred to this base. The terms upper and lower will be used to describe the tube aspositioned in Fig. l, andare not to be considered as limiting the position of the tube as actually' used.

Thecathodes are distributed around the base in 12V sets, each supported from its pair of leads 3 andi. Each pair of leads is arranged radially, with the mid-pointY of the pair approximately one-half wave length distant from the axis with respect to the nominal wave length of the tube, which, however, can be tuned through a reasonable range. The leads themselves are preferably constructed of hard-drawn copper tubing, and within each is an. inner tube, numbered. 'l `and 9 The oscilof even a relatively low peri or amplifier embodying the respectively. The innerk tubes terminate closely adjacent the upper ends of the leads 3 and 5, and

.water can be circulated through the outer tube and back through the inner, in order to eect cooling.

A..separate circular aperture is formed in the grid base I for each of the cathode leads. Downwardly from. each of these apertures a flange or collar II projects, which is contracted at its lower end and joined to an upwardly projecting thimble L3. The metal-to-metal joints between grid base and collar and collar and thimble are preferably hard soldered or brazed, to insure a gas-tight joint. Unless specied otherwise all of the other metal-to-metal joints which will later be described may be considered to be of this same character. Furthermore, the tube structure is almost entirely of metal, and where no specific material is mentioned the partsdescribed are to be considered as made of Kovarf copper, or other material which canV be thorougfhly outgassed. It will be understood that in building up the metal structure of the tube hard sol'ders of various materials and of various melting points are used, so that parts can be added successively without disturbing joints previously formed with solders of higher melting point. Details of this character are Well known in the art, and will generally not be discussed herein.

A short lengthof insulating tubing I5- is sealed to the upper end of each of the thimbles I3'. This, in turn, is sealed to a pair of cup-like cholres I'I and I9, mounted back to back with the cathode lead passing through the bottoms of the two cups, and secured thereto. The cathode leads terminate, just beyond the inner end of the cups I9, in"flags 2-I. The iilaments or cathodes proper 2'3 are bridged across between these flags, there being eightv fllaments to a set and each being mounted radially of the tube. The flags are, received in recesses` or slots in a cathode guard ring to be described later. The adjacent faces of the flags are dimensioned to form an open endedAV quarter wave length line, which by-passes suchr radio frequencyr energy as may be pickedV up by then-laments.

The guard ring structure isA massive, its alinement andV positioning with relation to the other elements of the tube are critical, and it must', in part, be adjustable. In order to meet these requirements, it is mounted on ay column carried axially ofthe grid base. A skirt 25 projects downward from an axial hole in the grid base I. Fitted within this skirt and brazed to its lower end is a tubular column 21 which carries on its upper end the annular guard. For convenience this is made in two parts; the support ring 29, which is secured directly to the column 2l, and the guard proper 3l., whichis a ring mounted at the periphery of the support ring 2Q. The recesses for receiving the flags ZI'extend entirely through the support ring 29, and nearly through the ring 3 I. The closure 33 remaining above the ags is slotted to. receive the laments 23, so that the upper surfaces ofthe laments lie substantially in the plane ofthe upper surface of the ring 3|.

.This structure is shown moreY clearly in Fig.` 6, which shows how the filament 23;, which is a flat short strip of thoriated. tungsten, bent downward at the ends toform a staple or horseshoe, is let. into a hole 32 in each of the flags, and, further, how the upper surface of the guard ring 3|' is formedY to bring the filament substantially level with its upper surface. Since the guard ring is biased to a relatively high negative potential, this conformation of the ring around the filament prevents side emission of the electrons and tends to focus them through the grid slots.

The grid cavity tuning mechanism is slidably mounted within the column 21. This mechanism comprises an outer tube 35 which ts snugly around a composite inner member. The structure of this member is best shown bythe cross section of Fig. 4. It consists of two similar substantially semi-cylindrical halves, which have their opposing faces milled out to form an H- shaped wave guide 31 when the two halves are brought together. Two additional parallel longitudinal slots 38 are milled in the outer surfaces of the sections, 36, and terminate in a circumferential channel 39. The channels 38 are covered by a member 49 which is turned down to conform with the cylindrical outline of the central member. The channel 39 is, as will be obvious, closed when the central column is inserted in itsouter tube 35, and cooling water is circulated in the passages thus formed.

The input of the wave guide 31 is through a horn 4|, by which it may be coupled to a wave guide of any desired characteristics. The horn is closed at its lower end by a glass window 43, which, sealing to the metal horn, forms a vacuum-tight joint.

The upper end of the wave guide is coupled into the cathode cavity by means of a coupling link 45, of approximately one-quarter wave length. It will be seen that this couples magnetically with the wave guide; a voltage loop is formed at its upper end, and this couples electrostatically with the cavity.

A iiexible annular diaphragm 41 connects between the inner periphery of the support ring 29 and the upper end of the tuning member 36, the adjacent inner end outer peripheries of the members 29 and 36 respectively being cut away to permit flexure of this diaphragm. The diaphragm forms an air-tight seal which permits the slight movement of the member 3S which is necessary for tuning.

Tuning is accomplished by means of screws 49 threaded into the end of the tuning member. These screws are rotated by means of pinions 5| driven by worms 53 and both mounted on a flanged disc 55 carried by the support tube 21 at its lower end.

The upper surfaces of the guard rings 3| and 29, the diaphragm 41, and the upper face 630i the tuning element 36, together form one of the two surfaces of the grid cavity 65. In order to provide separate D. C. biasing of the upper sur-- face of the grid cavity, this is formed by a cap which is separately supported from the grid base Collars 61 carrying thimbles 69, similar to those used to support the cathode leads, are carried in openings formed near vthe periphery of the grid base Insulating tubular struts 1| are sealed to the thimble 69 and to cup-shaped caps 13 at the upper end of these struts. Small discs 15, carried on the end of the cap 13, bear studs 11 to which the main grid cap support ring is bolted.

The support ring comprises an annular shoulder 19, provided with an outer tubular portion or skirt 8|, which surrounds the supporting colums, and an inner tubular extension to which the grid cap is secured.

The grid cap comprises a tubular portion 85 which is essentially an extension of the tubular member 83, closed by a nearly flat plate 81. Just from it by a very small distance, is a depending skirt 89, which is about -a quarter of a Wave length long. The outer portion of the plate 81 conforms closely with the upper surface of the grid ring 3|, and is slotted immediately above the laments 23 to form grids of the same character as were described in the Sloan and Marshall patent above referred to. The central portion, above -the'ring 29 and the tuning element 36, is truly flat. This central portion of the plate 81 is made of two layers to permit the formation of an annular channel 88 for the circulation of cooling water. Water is supplied through a pair of manifolds 9| which circle the grid cap immediately above the shoulder 19. Capillary nickel tubes 93, leading from the manifolds, are

inlaid in grooves in the grid cap and so lead into the channel, passing under the grid bars as is shown inv Fig. 6.

The cavity between the two halves of the grid cavity structure oscillates substantially as though the gap between the two did not exist. It will be seen that the path around the skirt 89 is substantially one-halfl wave length long. It forms what is essentially a coaxial transmission line, and since it is closed at its end it acts like a s'hort circuit at the outer edge of the ring 29. The transmission line branches at the end of the dependent skirt 89. The lower L portion is a quarter wave length open ended line, and viewed from the point where it branches it appears like a short circuit, whereas the upper portion appears from this point very nearly like an open circuit. Accordingly, the voltage drop at the point of juncture will almost entirely be expended across the open circuited portion, whereas the short circuit portion will act as a by-pass. As a result, very little energy will escape from the grid cavity.

The cavity oscillates most readily at the TMo,3 mode, with voltage nodes on each side of the dependent skirt 89 and between the axis and the filaments, and loops at the axis, the filaments, and at the lower edge of the skirt. At this mode little powerjcan escape through the gap between the guard ring and grid, owing tothe relative impedance between the paths, but at other TMO, frequencies departing materially from that desired, the excitation by the grids becomes less eilicient, and the paths up and down from the edge of the skirt 89 approach the same impedance, so that power is radiated through the gap and the 'oscillations damp out. The resonator is nota cavity, in the ordinary sense for such modes, though it acts like one for the desired mode.

The closure near its periphery is virtual, not actual, and it wouldrtherefore be nearly if not quite as accurate to consider the cavity as closed at the voltage node at the inner base of the skirt 89 and oscillating in the TMW mode. The potential distribution between the axis and the skirt is the same from either point of view.

This completes the description of the cathode and grid structures, all of which are supported from within the grid base The anode and accelerator grid structures are supported externally of theY grid base I. A ring 95, depending from the lower outer edge of the grid base, carries a skirt 91 to which is brazed a sealing ring 99. Sealed to this is the main envelope IUI of the tube, a cylinder of hard glass which is bulged slightly at vthe center to carry it away from the principal electrostatic stresses between the high potential portions of the tube. At its upper end 7 the envelope is sealed to a second 'sealing ring |03 which is secured through a skirt |95 to an ac celerator grid support ring |01. Through an intermediate ring |09 this carries a support tube to which the accelerator grid proper is secured. This comprises a disc i|3, whose lower surface is conformed to the outer surfaceV of the grid 81. it is flat except for an annular bulge -immediately above the filaments.. This portion of the accelerator grid is slotted, the grid function being supplied by the. bars left between the slots. Like the control grid, the central portion of the accelerator grid is formed in two layers to provide a cooling channel ||5, fed by a pair of manifold tubes ||1 and capillary feeders H9.

The space between the accelerator grid and the control grid does notv form a resonant cavity at the operating frequencies of the tube. At these frequencies the flange |2|, which surrounds the accelerator grid, forms with its inwardly project:- ing skirt |23 a quarter wave choke which oifers a very high impedance at the periphery of the cavity, across which the maximum voltage drop wili' occur, and the space between the. skirt |23 and the grid cap 85 forms ahy-pass. The high impedance. at the operating frequency prevents anymaterial build-up of voltages thereat, while the by-pass prevents the escape of power through forced oscillation. At' other frequencies the bypass section is notl operative as such and itself contributes to the damping.

The anode structure is supported from the acceleratorV grid support ring |01 by means of an insulating tube |25 secured to the support ring by a seal ring |21. A second seal ring |29 connects with the anode support ring |3| which carries the series of concentric tubes forming the anode structure. Of these tubes the outermost |33,'the intermediate tube |35, and the inner tube |31, are spaced so that between them is formed a channel through which cooling water may be circulated through the input and output ports |39 and lill respectively. A massive copper outer anode ring |43' is secured to the end of tube |33. An inner anode ring |45 is secured to the tube |31. Between the two, and mounted on tube |35, is the anode proper.

The anode structure can best be appreciated by reference to Fig. 3. In constructing it, a solid copper hoop |46 is provided, into one edge (the lower, as shown in the figure) of which is milled a succession of narrow V-shaped slots equal in number to the filaments; i. e., in the present instance, 96 such slots. After the completion of this operation short tubular hoops, thinner than the hoop |136, are fitted within and without the circumference of hoop |45, and are brazed in place. From the other, or upper side of the composite hoop thus formed, another succession' of slots for passage of water is milled through all three of the members forming the hoopr and the whole is then brazed onto a ring |49, which in turn is fastened to the tube |35. The lips |50 and |52 of the inner and outer anode rings are then brazed t the hoops |51 and |48 respectively, which completes this portion of the anode. The whole forms an annular ridge on the anode structure1 which faces the filaments, the V-shaped grooves being opposed to the filaments themsel-ves. Thev slots which extend through members |46; HH and |43 connect the channels between tubes |33 and |35 and between |35 and |31, respectively, soV that the water which is circulated through these channels passes in close proximity with the anode surfaces bombarded by the electron flow from thecathodes and keeps them cool.

The'` shape of the anode cavity, particularly in the portion where the electron iiow from the filament is received, is important. The

Vvoltage between anode and cathode is high, but

there is still a considerable secondary emission of electrons where impact occurs. insofar as the secondaries are emitted within the V'shaped slots in the anode, the emission vis largely trapped and takes nov further part in the discharge, but where secondaries are released from the ridges between the slots and from the adjacent portions of the anode structure the results may be serious, especially since the ratio of secondary electrons to primaries may be considerably greater than unit. If these secondary electrons have a transit time which causes them to resonate with the desired frequency their numbers may build up by geometrical progression until the oscillating electrons effectively form a short circuit at what is desired to be a potential loop, with the result that the tube can actually cease to function for this reason alone.

A defocusing fieldA is thus formed which causes secondary electrons to migrate away from the primary electron stream, toward loci on either side thereof where the paths across the cavity are longer and the oscillating voltages are less. Both of these effects increase the transit time of the electrons and cause them to fall out of resonance Ywith the oscillating field so that they no llonger cause further sec ondary emission and are eventually picked up by one of the cavity'surfaces. This feature may not be particularly important when there is a large D. C. potential difference between accelerator grid and anode to sweep out the secondaries, but when the tube is operated with these elements at or'near the same potential it becomes a necessary condition of operation.

The outerA ysurfacefof the block |45 is provided with circumferential and radial groovesl |53 and |55 respectively, which form chokes and by-passes for defining the extent of the plate cavity |59 and determining its modes of oscillation as has heretofore been described in connection with the grid cavity.

The anode tuning mechanism is slidably mounted within the tube |31 and is a substantial duplicate of the mechanism used for tuning the grid. It comprises an outer tube |B| and an inner column |63 with a cooling channel |65 formed between them. The sliding and stationary portions of the mechanism are joined by an annular diaphragm |61 which performs the same function 'as the diaphragm d'1 of the grid tuning mechanism. Tuning is accomplished by means of screws |63 carried by the block |59 mounted on the end of tube |31. The screws are rotated by' means of pinions I1'| and worms |13. v

Power is Aabsorbed. from the plate cavity by a quarter wave link |15 coupling with the wave guide |11 which extends axially through the anode tuning structure and terminates in a horn |19 and a window (not shown) similar to theA grid window d3. Y

In Fig. 5 there isrshown a modified cathode and grid structure which may be substituted for that shown in Fig. l, using an identical anode structure. The structure of Fig. 5 possesses some disadvantages in comparison with that shown in the first figure, but it is enough coupling between the output easier to construct so that in many cases it would be the choice.

In the modified form of the device the grid base I.is reduced to a narrow ring I', which supports the grid cap on struts Il in precisely the same manner as in the form of the device first described. The various elements of this structure which are identical with like elements of Fig. 1 are identified by the same reference characters as those in the initial figure.

Rising from the inner edge of the base ring l. is a-thin cylinder 20|, carrying at its upper end anl outer filament ring 203. Concentric within this ring is an inner iilament ring 205, spaced from the ring 203 by a narrow gap which forms a quarter lwave open-ended line. Y'I'he filaments 23 are bridged across between the two rings, all of the filaments in this case being connected in parallel. Fig. 'l shows a transverse section through the filaments, indicating how the rings 203 .and 205 are cut away to reduce the capacity between the filament rings and the vgrid cap 81. A pair of concentric tubular flanges 201 and 209 depend from the filament rings 203 and 205 respectively, being spaced some -distance `back from the gap betweenA the two rings. the two anges 201 and 209.

A support tube 21 is bolted to the inner periphery of the ring 205. Thisr is so `sealed as to make a. gas-tight joint, and as the structure within it is precisely the same as that within the support tube 21 the description is not repeated here.

Current is carried to the two filament rings through a plurality of copper tubes 2I3, 2I5, which are brazed to the filament rings and carry cooling water as Well as current to the filaments.

It will be noted that in this instance the lilament supporting structure forms the lower portion of the grid cavity. This possesses the disadvantage that the lateral focusing effect of the guard structure in the first figure is absent, and some electron emission is accordingly wasted. On the other hand, the structure is so much simpler and cheaper to build that some filament efficiency may be sacried in order to Again these advantages.V n Where a .tube of the character of those here described is driven by an' external oscillator, and

acts merely as an amplifier, there is less dif-- culty likely to be experienced through oscillation in Vundesired modes. Where, however, the device is self oscillatory and is driven by and the input, parasitic modes of oscillation could practically destroy the usefulness of the steps here taken to prevent it.

'For selfoscillation to occur at `any frequency the electron coupling between the cavities must occur in the neighborhood of voltage loops in both; in the grid Vcav-ity to modulatey the electron stream, and in the plate cavity to absorb energy from the stream by retarding the electrons. Both cavities must respond to the same frequency, and the feed-back through the eX- ternal circuit must be in the proper phase.

In an axial filament tube these three conditions coincide in any important degree only withl respect to the TMm modes; the frequency discrepancies between the one of these modes de.- sired and others of the same class practically insures that any feed-back of the latter will vbe out of. phase,y and the voltage loopsof the TML",

A glass seal 2|| joins tube but for the l Vhave almost any value,

Voltage loops in some of these latter inodes,`

however, notably the TM2,3 and TM3,3, would normally occur not far from the radius at which the filaments of thev instant tube arelocated, and the frequencies can be near enough to the TMm frequency to get feed-back in the proper phase to cause oscillation. Not all of the grid slots will be effective at these modes, for nodes land loops alternate around the grid-slot circle, the loops themselves alternating in polarity, and the gridslots located at the loops are suiiicient to cause the trouble, especially since the coupling with the output circuit is poor at these modes and hence provides little damping.

These effects are suppressed in accordance with this invention by three counter-measures: relative detuning of the undesired modes in the two cavities, relative displacement of the voltage loops therein, and damping.

The first two of these measures are contributed the stepped grid cavity alters both the frequency and the lield pattern of that'cavity as compared to the anode cavity.

Also effective, however, are radial slots 2li', formed in the end plate or plates of one (or both) of the cavities. For best results the slots are odd in number, as intermediate slots between the odd numbered ones usually have little elect. They are also more effective if spaced somewhat irregularly.

Slots of this character have very little effect on the TMO, modes, for they are parallel -to the current iiow, and hence increase the inductance of the cavity slightly. will cut across the lines of current iiow of the normal pattern of any of the more prominent TMm modes, in any position within the cavity which the modes might assume.

Slots across the lines of iiow vwill not prevent such iiow at the frequencies here considered. The sides of the slots, passing through the cavity wall, can be considered as the conductors of transmission lines, and the resultant effect clepends both on the thickness of the cavity walls and on what is outside of the cavity. In any practical tube the thickness will be such that the transmission line will be of less than a quarter Wavelength, and hence will appear in the resonant system like a'series capacity. This capacity may depending on wave length, depth of slot, etc. If the impedance it offers to the oscillations is high enough it may divert the currents in to a longer path and increase the current density in that path, thus raising the effective inductance and lowering frequency, or, again, it may act as a capacity to raise the frequency of a mode having any specified number of voltage maxima. Since the number of such maxima is always even, and since the slots are somewhat irregularly spaced, some of the circulating current paths will 'be cut by more slots than will others, and the effect is to distort the oscillation pattern and make it asymmetrical, so that there will be fewer locations around the ring of grid slots where maximum grid control and maximum power of the plate cavity to absorb energy coincide. y Y

Most important, if the slots terminate in the pen they will radiate the power of the parasitic modes, and damp out their oscillations "before,

Several of them, however,v

they can build up. They have no Vsuch damping y limited number of the possible modes are probable, i. e., excited as readily as the desired one. Some of these probable modes can be prevented entirely, but where mutually detuned, as between cavities, and where so detuned no appreciable feed-back will occur between anode and grid cavities, and hence even if such modes are initiated in either cavity they will immediately damp out and cease to exist.

It should be noted that where the terms half wave length and quarter wavelength are used, what is meant is the distance betweentwo voltage nodes or loops or between a voltage loop and a node, in the particular structure mentioned. These distances will not, in general, be one-half or one-quarter of the wave length in free space of the frequencies at which the device is operating. They will instead be 'dependent on phase velocities which in general -will be greater than the free space wave length of a `corresponding wave. On the other hand, where inductance or capacity are high, as they may be in a coaxial structure, for example, the fractional wave lengths may be shorter than the free space half and quarter wave lengths.

One point which is brought out in the prior patent previously referred to in this application but which should also probably be mentioned here, is that the quarter wave choices and bypasses which have frequently been 'mentioned are of quarter wave length only at the mid-frequency of the band over which the device tunes. At other frequencies not too far distant from this, while they do not act as infinite or Zero impedances, nevertheless the impedances that they ofier to thesefrequencies are very high and very low, and the division of potential across them is such that very little power is wasted by radiation from the parts ofthe device to which they are applied.

It is to be understood that the principles which are embodied in the tube which has been described are not limited to this specific device, but are of general application. I Vtherefore do not wish to be bound by any limitations in this specification except those which are specifically expressed in the appended claims.

I claim:

1. In a combination with a microwave device, a cavity resonator having a transverse dimension greater than the wave length at which it operates and means spaced substantially an integral number of half wavelengths from the axis of said resonator for exciting electrical oscillations therein, said resonator having formed therein a plurality of substantially radial slots to intersect' the currents of parasitic modes of oscillation .of the resonator as a whole.

2. Apparatus in accordance with claim 1 wherein said exciting means comprise `a plurality of circularly arranged exciting units, disposed approximately one-half Wave length of the desired mode of oscillation'from-the axis of the resonator. Y

3. Apparatus in accordance with claim 1 wherein said slots are odd in number.

they cannot be they can be 4. Apparatus in .accordance with claim '1 wherein said slots are odd in number land asymmetrict-illyY arranged. Y

5. Apparatus in accordanceV with claim` 1 wherein said slots are asymmetrically arranged.

6. Apparatus in accordance with claim. l wherein said exciting means comprise a plurality of radially disposed filaments centered approximately one-half wave length of the-desired mode of oscillation from the axis of said resonator.

7. A microwave generator comprising a plurality of oscillation generating units of relatively low power, and means mutually coupling said units to give a combined high power output-at substantially a single frequency, said coupling means comprising a cavity resonator the diameter of which is larger than its operating wave length, and said units being circularly mounted at a voltage loop within the cavity of said resonator spaced from the axis thereof.

8. Apparatus in accordance with claim 7 wherein radial slots are formed -in said resonator between said vunits to suppress oscillation therein at modes other than the fundamental.

9. Apparatus in accordance with claim 1 wherein the axial length of said resonator is materially less than its transverse dimension.

10. A tetrode for operation at microwave frequencies comprising -a cathode, a plate structure and a grid structure, and cavity Vresonators for each of said structures having transverse dimensions larger than the wave length of the desired frequency vof operation, said resonators being mutually coupled through apertures in said grid structure spaced from the axis of said cavities by substantial-ly one-half of the wave length therein of said frequency'of operation.

11. A cavity resonator having a transverse dimension greater than one wave length of the desired operating frequency, a plurality of exciting elements spaced'from the axis of said resonator substantially one-half 'wave length at said frequency within the resonator, and means Within said resonator for Vsuppressing parasitic oscillations within said resonator at frequencies adjacent said desired frequency.

12. In a vacuum tube for operation at microwave frequencies, a grid cavity resonator, an anode cavity resonator mounted coaxially therewith, each of said resonators beingdimensioned to oscillate at the. same desired frequency, a plurality of exciting 'elements coupling said resonators and disposed at a potential loop at thedet sired Inode spaced from the axis of said .resonators, and mutually detuning means as to undesired modes of oscillation formed withinat least one of said resonators.

13. Apparatus in accordance with claim 12 wherein said detuning means comprises .astep formed in one of said cavities at a'voltage node therein ofthe desired frequency. t

DAVID sLo'AN.

References Cited vin the :iileof thispatent UNITED STATES PATENTS Nnizler Name Date A ,l ,538 Potter July 5 1938 2,242,275 varian May 20 1941 2,398,162 Sloan Apr. `9, 1946 2,439,387 Hansen et al. Apr. 13,1948 2,464,984 Lobera...V Mar. '22, 1949 

